body {
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-sizing: border-box;
    position: relative;
    z-index: 0;
    width: 100%;
    max-width: 640px;
    min-height: 100%;
    margin: 0 auto;
    margin-top: 0px;
    margin-right: auto;
    margin-bottom: 0px;
    margin-left: auto;
    overflow-x: hidden;
    box-shadow: 0 0 10px rgba(0,0,0,0.3);
}

user agent stylesheet
body {
    display: block;
    margin: 8px;
}